Environmental drivers of crocodyliform extinction across the Jurassic/Cretaceous transition

Jonathan P Tennant,Philip D Mannion,Paul Upchurch

doi:10.1098/rspb.2015.2840

Abstract

Crocodyliforms have a much richer evolutionary history than represented by their extant descendants, including several independent marine and terrestrial radiations during the Mesozoic. However, heterogeneous sampling of their fossil record has obscured their macroevolutionary dynamics, and obfuscated attempts to reconcile external drivers of these patterns. Here, we present a comprehensive analysis of crocodyliform biodiversity through the Jurassic/Cretaceous (J/K) transition using subsampling and phylogenetic approaches and apply maximum-likelihood methods to fit models of extrinsic variables to assess what mediated these patterns. A combination of fluctuations in sea-level and episodic perturbations to the carbon and sulfur cycles was primarily responsible for both a marine and non-marine crocodyliform biodiversity decline through the J/K boundary, primarily documented in Europe. This was tracked by high extinction rates at the boundary and suppressed origination rates throughout the Early Cretaceous. The diversification of Eusuchia and Notosuchia likely emanated from the easing of ecological pressure resulting from the biodiversity decline, which also culminated in the extinction of the marine thalattosuchians in the late Early Cretaceous. Through application of rigorous techniques for estimating biodiversity, our results demonstrate that it is possible to tease apart the complex array of controls on diversification patterns in major archosaur clades.

Highlights

Crocodyliforms are a major group of pseudosuchian archosaurs that include living crocodylians
These parameters can be broadly divided into two categories: (i) those that predict biodiversity to be driven by sampling-related artefacts, i.e. non-marine rock outcrop area, and numbers of fossiliferous marine formations [19]; and (ii) those that represent environmental proxies, independent of sampling, i.e. eustatic sea level [38,39]; temperature (d18O) using [40], and the independent dataset presented in [11]; the global carbon (d13C) and sulfate (d34S) cycles [40]; weathering rates (87Sr/86Sr) [17,40]; as well as an estimate of global subsampled marine invertebrate biodiversity [17], which we use as a coarse proxy for potential food resources for marine crocodyliforms
This might have been important in releasing ecological pressure, resulting in opportunistic replacement by marine macropredaceous groups, such as the diversification of plesiosaurian [14] and shark [61] lineages immediately after the J/K boundary, and the subsequent diversification of pancryptodiran and pleurodiran turtles [62,63]. The radiation of these clades suggests that there might have been broader ecological shifts occurring in semi-aquatic to shallow marine reptile faunas [64], and the occupation of high tier predatory niches by new groups was likely an important factor in suppressing the recovery of marine crocodyliforms. This pattern is distinct from that observed in continental crocodyliform ecosystems: there we see a drop in biodiversity followed by a rapid recovery and subsequent radiations (Eusuchia and Notosuchia) during the Early Cretaceous [2,8], representing a faunal turnover in non-marine crocodyliform faunas as ecological pressure was released following the J/K boundary decline

Summary

Introduction

Crocodyliforms are a major group of pseudosuchian archosaurs that include living crocodylians. SQS was applied to our marine and non-marine genuslevel occurrence datasets for each time interval to provide an estimate of global subsampled taxonomic richness, using two methods (each using our two binning strategies; section SI 3 in [26]). We extracted a range of sampling proxy and environmental data from the primary literature (section SI 1 in [26]) to test whether extrinsic factors were the drivers of crocodyliform biodiversity dynamics These parameters can be broadly divided into two categories (see table S3 in section SI 1 in [26]): (i) those that predict biodiversity to be driven by sampling-related artefacts, i.e. non-marine rock outcrop area, and numbers of fossiliferous marine formations [19]; and (ii) those that represent environmental proxies, independent of sampling, i.e. eustatic sea level [38,39]; temperature (d18O) using [40], and the independent dataset presented in [11]; the global carbon (d13C) and sulfate (d34S) cycles [40]; weathering rates (87Sr/86Sr) [17,40]; as well as an estimate of global subsampled marine invertebrate biodiversity [17], which we use as a coarse proxy for potential food resources for marine crocodyliforms. The relative fit of each variable was assessed using the sample-size corrected Akaike information criterion (AICc) and standard correlation tests (see section SI 3 in [26] for detailed protocol)

Results

Discussion

Findings

Conclusion